Inkjet technology and the application of inkjet technology to printing is now well known. Ink is delivered through nozzles from the cartridge. Resistors underneath each individual nozzle locally heat the ink which is then ejected through the nozzle and onto the paper. Relative nozzle spacing is a critical parameter. Errors in nozzle to nozzle placement lead to distortions in printed pattern discernable to the eye. The axis of each nozzle must point in same direction to within 1 degree or better. Nozzle shape must also be controlled both for consistency and exact shape so the subsequent ink flow has the desired form. Typical materials for inkjet nozzles are polyimide, stainless steel, and silicon.
The prior art for manufacturing inkjet nozzles can be broadly divided into 4 categories. They are mechanical punching, chemical etching, laser machining with contact masks, and laser machining with imaging systems. We now discuss each in turn.
Mechanical punching is limited to relatively soft materials such as polyimide, and large holes, &gt;0.010". Since inkjet nozzles for future generations are generally smaller than that, punches have little future for nozzle manufacture. Wear on the punches also alters the precise nozzle profile, necessitating frequent resharpening to maintain the profile.
Chemical etching is another technique for nozzle production. By chemical etch we include both wet chemical etch and reactive ion etching (RIE). An etch block is applied to both sides of the surface leaving openings at the nozzle locations. The material is subsequently etched.
FIGS. 1A-1F illustrate the processing steps involved in nozzle fabrication for wet chemical etch. FIG. 1A shows the substrate 11 which is to be patterned with nozzles. FIG. 1B has photoresist 12 applied and then FIG. 1C exposed as at areas 13. Exposed areas 13 are then developed FIG. 1D, leaving openings 14 in the patterned photoresist. In the next step, FIG. 1E material is submersed in a wet etch bath and removed by chemical action in regions 15. After stripping the resist, FIG. 1F, we are left with nozzles 16 in the material. This 6-step process has been illustrated for wet chemical etching where both sides of the material are simultaneously etched. If only a single side is etched, either wet chemical or RIE, the number of steps would be identical.
The large number of steps in this process contributes to yield loss. Controlling the wall slope is very difficult for wet chemical etch. For RIE, wall slope control is generally possible but the batch nature and relative high cost of the equipment generally preclude RIE use. A further difficulty with aqueous chemical etch is the dimensional deformations it induces in polyimide due to water absorption.
Another technique is laser machining utilizing conformal masks. This method is described as it relates to the manufacture of wiring patterns in "Generation of Blind Via-Holes for a High Density Multi-Chip-Module Using Excimer Lasers", F. Bachmann, Materials Research Society Symp. Proc. Vol. 158, 1990. Adapting this technique to nozzle manufacture would entail the 9 step process illustrated by FIGS. 2A-2J. The substrate 21 is coated with photoresist 22, FIG. 2D, exposed in regions 23, FIG. 2C, and developed at regions 23, FIG. 2D. Next, FIG. 2E, a seed layer of metal 27 is sputtered onto the tape and subsequently FIG. 2F plated up to fill in the depressions 28, where the photoresist has been removed. Ideally, plated up metal layer 28 would not cover photoresist 29, but to insure all depressions are filled, some overplating is desirable. Metal layer 28 is then etched back FIG. 2G to expose the photoresist openings 29. Next FIG. 2H, the photoresist is stripped opening up openings 31 in plated up metal 28. A laser beam is then scanned over the surface, plated metal 28 serving as an etch block and openings 35 controlling location and size of the nozzles. The result of laser ablation are nozzles 36.
If instead of polyimide, the material is tougher, like stainless steel, etch barrier would need to be considerably thicker, or, steps 2A-2H must be repeated multiple times until the nozzles are made; this multiplies the total number of steps by the number of repeats. The large number of steps for the basic process contributes to yield loss.
Another disadvantage of this process is the inefficient use of laser light. Since typically &lt;1% of the area has openings 35 for nozzles, a laser beam sweeping over the surface as at FIG. 2G wastes &gt;99% of the light since most of it is intercepted by etch block 30. This inherent inefficiency leads to lower process throughput. Also, the large area scanned by the laser beam coupled with the inherent inefficiency of the process leads to local heating of material which can result in thermal damage. A further draw back of this process is that the openings 35 permit limited latitude for wall shape control since they can modulate the intensity pattern only in an on/off fashion; no variation of intensity level (grey level) is possible.
Another technique for manufacturing inkjet nozzles are projection imaging systems for direct etching. A mask in the form (possibly magnified) of the desired image is projected onto the workpiece by imaging optics. The imaging optics are low (de)magnification, typically 1.times.-5.times., and have a field of view larger than the pattern of interest. This is a single step process. Nozzle substrate is directly machined forming nozzles in the locations dictated by the mask. This approach is described in "Excimer Laser Based Microstructing Using Mask Projection Techniques", U. Aarbach, H. Kahlert, Lambda Highlights, No. 40, Pg. 2, April 1993, and "Patterning of Polyimide Films with Ultraviolet Light", U.S. Pat. No. 4,508,749. J. Brannon, J. Lankard, April 1985. Because of the small open area of most nozzle arrays, only a small fraction of the light incident on the mask ultimately performs useful work removing material for nozzle. Said differently, the actual area of the nozzles is small compared to the imaged area of the mask, so that when the mask is illuminated only the light incident on the part of the mask defining the nozzles is actually used; the rest is thrown away. The result is inefficient use of light and therefore lower machine throughput for a given laser power.
Another limitation of imaging techniques is field of view. For 1.times.-5.times. reduction systems, fields of view &lt;5 mm are relatively common, while larger fields of view become increasingly difficult to obtain. In addition to the difficulties associated with obtaining large fields of view, the imaging optics must be designed to withstand the high peak and average power levels associated with direct machining. Optical coating damage and bulk changes in refractive index are problems associated with this approach. Dielectric 1.times. masks, as described in "Excimer Laser Based Microstructing Using Mask Projection Techniques", U. Aarbach, H. Kahlert, Lambda Highlights, No. 40, Pg. 2, April 1993, and "High Energy Laser Mask and Method of Making Same", U.S. Pat. No. 4,923,772. S. Kirch, J. Lankard, K. Smith, J. Speidell, J. Yeh, May 1990 must be capable of withstanding the power levels and are therefore limited in their scope of use. Dielectric masks are further limited to allowing only light intensity passing to take only two values, that is no intensity what so ever and the full illumination intensity. Having only two values available for the intensity level allows for limited latitude in adjusting the intensity profile on the workpiece, thereby influencing the nozzle wall slope. Uncoated 1.times. masks as described in "High Power Phase Masks for Imaging Systems", A. Smith, R. Hunter. U.S. patent application Ser. No. 07/833,939 filed Feb. 10, 1992 now U.S. Pat. No. 5,328,785 issued Jul. 12, 1994. and as used in Apparatus and Process for Fine Line Metal Traces, U.S. patent application Ser. No. 08/058,906 filed May 6, 1993 now U.S. Pat. No. 5,364,493 issued Nov. 15, 1994, can withstand substantially higher fluences, but require special, low angle condenser optics. Still, at the highest fluences (.sup..about. 40 J/cm.sup.2 @248 nm for stainless steel), even these masks are damaged in a 1.times. imaging geometry. To minimize mask damage problems, reduction systems--typically 5.times.--are used. Then, with 25.times. less power per unit area on the mask, mask damage problems are minimized but mask cost is increased because 25.times. more area is written. However, going from a 1.times. to 5.times. lens design substantially increases the difficulty in both design and fabrication of the imaging optics. Practical limits on available mask size also limit field size.